Brushless Direct Current (BLDC) motors are a type of electrical motors having multiple phase windings (e.g. two or three or more) and a permanent magnet as a rotor (e.g. a bar magnet or a multi-pole magnet). BLDC motors do not use brushes for commutation, but instead they are electronically commutated. This implies, however, that the drive circuit must know the relative position of the rotor with respect to the stator, in order to direct the magnetic field for exerting torque on the rotor. One known way to do so is by adding positioning means to the motor, such as e.g. optical positioning means or magnetic positioning means.
The present invention, however, is related to a category of motors without such additional positioning means, known as “sensor-less” motors. In such motors, the relative position of the rotor (with respect to the stator) is typically determined based on measurements of the so called “back EMF (BEMF)” or “back voltage”. This method is especially suited for three-phase motors, whereby two of the three phase windings are powered by block waves, while the third winding is left unpowered. The back EMF can then be measured over this third winding. By monitoring the back EMF, the motor drive circuit can derive the relative rotor position, and adjust the commutation accordingly. The BEMF can also be used to determine whether the rotor has stopped, e.g. because it is blocked or has reached an end stop.
A problem with such motors is that the torque is limited, because only two of the three windings are powered. Another problem is that the torque is not constant, which results in increased noise and vibrations. Both of these problems are solved in the art by using motors having windings arranged such that they would create a sinusoidal-alike BEMF voltage (if they were mechanically driven externally). These motors should ideally be driven by three simultaneous sinusoidal waveforms, but have the problem that there is then no winding anymore for monitoring the BEMF. Some solutions are proposed in the art, whereby one of the windings is temporarily opened, so that the BEMF can be monitored (albeit briefly).
U.S. Pat. No. 7,432,682(B2) describes a different approach, not based on the back-EMF, but on the sum of the currents flowing through the phase windings. The US patent describes how phase-shifted waveforms (replicated herein as FIG. 2) are applied to the phase windings, using a drive circuit (replicated herein as FIG. 3). The sum of the currents is then measured over a resistor R1, which signal is low-pass filtered by an RC-filter, and then sampled. Then statistical analysis is applied to the samples, so as to determine the “variance” of the sample-values. When the motor is running at normal speed this variance is relatively low, but when the motor has reached its end position, the variance is large. This phenomenon is used to determine whether the motor has stalled. However, this technique is complex.